Narrative:

Just after takeoff; got an IRS/position disagree light. At 400 ft; called for LNAV and FD bars disappeared. LNAV info looked incorrect so we notified ATC we were unable to utilize the RNAV departure. ATC gave us vectors toward trm. Out of 10;000 ft I called for the climb checklist and noted nothing abnormal. As we reached FL230; the intermittent cabin altitude warning horn sounded. Preformed memory items; O2 mask on 100%; and communication established. Called for checklist; notified ATC; and declared an emergency. Continued to run the checklist; and not seeing an immediate cabin change; we asked for and initiated an emergency descent to 7;000 ft. We called the flight attendants; had them secure the cabin and asked ATC for a clearance to return to our departure airport. We then got the cabin pressure under control and informed the passengers we were returning. ATC headed us back to lax; and we then asked for holding or vector to slow things down [while we checked with dispatch and maintenance on overweight landing data due to being 7000 to 8000 pounds over maximum structural landing weight. We landed uneventfully; filled out logbook; and contacted dispatch; maintenance; and the chief pilot on duty.with the things that were happening; it was an early indicator that the aircraft was not sure it was in the air. Closer and more frequent checks of the cabin pressure could have been done.

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Original NASA ASRS Text

Title: When confronted with multiple system anomalies--including an FMS position error and the failure of the pressurization system--shortly after takeoff; the flight crew of a B737-300 declared an emergency; initiated an emergency descent from FL230; and returned to their departure airport.

Narrative: Just after takeoff; got an IRS/POS Disagree light. At 400 FT; called for LNAV and FD bars disappeared. LNAV info looked incorrect so we notified ATC we were unable to utilize the RNAV departure. ATC gave us vectors toward TRM. Out of 10;000 FT I called for the Climb Checklist and noted nothing abnormal. As we reached FL230; the intermittent Cabin Altitude Warning horn sounded. Preformed memory items; O2 mask on 100%; and communication established. Called for checklist; notified ATC; and declared an emergency. Continued to run the checklist; and not seeing an immediate cabin change; we asked for and initiated an emergency descent to 7;000 FT. We called the Flight Attendants; had them secure the cabin and asked ATC for a clearance to return to our departure airport. We then got the cabin pressure under control and informed the Passengers we were returning. ATC headed us back to LAX; and we then asked for holding or vector to slow things down [while we checked with Dispatch and Maintenance on overweight landing data due to being 7000 to 8000 pounds over maximum structural landing weight. We landed uneventfully; filled out logbook; and contacted Dispatch; Maintenance; and the Chief Pilot on duty.With the things that were happening; it was an early indicator that the aircraft was not sure it was in the air. Closer and more frequent checks of the cabin pressure could have been done.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.